This invention relates to silicone elastomers based on platinum-catalyzed, hot-curing organopolysiloxane compositions which are characterized by particularly low compression set after crosslinking and to a process for the production of these elastomers.
In the test according to DIN 53 517, conventional organopolysiloxane elastomers can have a compression set of up to 95% when they are exposed for a while to temperatures above 150.degree. C. However, in many applications, for example as seals, organopolysiloxane elastomers have to be able to withstand temperatures of more than 300.degree. C. for prolonged periods without any excessive increase in their compression set.
For compounds based on diorganopolysiloxanes which can be crosslinked to elastomers by organic peroxidic compounds, methods of obtaining particularly low compression set values without heating are described in the prior art, cf. for example U.S. Pat. Ser. Nos. 3,261,801, 3,468,838 and 3,865,778 which mention such additives as barium peroxide, barium oxide or magnesium oxide for obtaining a particularly low compression set after vulcanization.
DE 2 911 352 describes the use of at least one basic inorganic solid hydrophobicized by treatment with organosilicon compounds for the production of elastomers which show a particularly low compression set after heating in the presence of air.
Another process for the production of organopoly-siloxane compounds having a reduced compression set which can be cured to elastomers is described in EP 0 010 708. In this case, additions of 0.1 to 8% by weight, based on the mixture as a whole, of polycarbodiimide in the form of polycarbodiimide/polysiloxane copolymers improve the compression set.
As already mentioned, the known processes described above for improving the compression set of polysiloxane elastomers are applicable to any compounds which can be crosslinked to elastomers by organic peroxidic compounds. In recent years, however, organopolysiloxane compounds crosslinked by a hydrosilylation reaction of vinyl-containing siloxanes have been acquiring increasing interest. It is known that this hydrosilylation reaction is catalytically accelerated by a number of platinum compounds.
Like peroxide-crosslinked systems, platinum-catalyzed addition systems of this type can have compression sets of up to 75%. It would be desirable to increase the compression set for a number of industrial applications of these addition-crosslinked elastomers.
However, the processes recommended for peroxide-crosslinked organopolysiloxane compounds to reduce the compression set of their vulcanizates fail when applied to platinum-catalyzed addition systems.